The present invention relates to probes for measuring electrical conductance and more particularly relates to probes for determining water concentrations in fluids. Also, the present invention relates to methods of determining water concentration in the refrigerant of a hermetic vapor compression refrigeration system.
Water concentration measurements are commonly needed for a variety of purposes. For example, when operating a hermetic vapor compression refrigeration system it is desirable to detect water concentration in the refrigerant of the refrigeration system. A relatively high water concentration in the refrigerant may indicate a problem such as a water leak in a heat exchanger tube. In certain situations it is desirable to detect and correct such a problem as soon as possible. Also, relative humidity sensors are often used in controls for equipment such as air conditioning and refrigeration machines.
It is especially desirable to measure water concentration in a fluid such as a lubrication oil for a compressor drive train of a hermetic vapor compression refrigeration system. This is true because typically such a hermetic vapor compression refrigeration system has an oil lubrication system for the compressor drive train wherein refrigerant from the refrigeration system is constantly in contact with the oil of the lubrication system. Typically, in such a refrigeration system, an oil reservoir for the lubrication system is located in the compressor transmission drive housing which is vented to the suction side of the compressor to reduce windage losses in the housing. Refrigerant is continuously circulating through the compressor drive housing from the refrigeration system and there is constant contact between the refrigerant and the oil. Water and other contaminants in the refrigerant are deposited in the compressor drive lubrication oil because of the contact between the refrigerant and the oil. Since the portion of refrigerant in contact with the oil is constantly changing, the amount of contaminants in the oil tends to be an average of the contaminants in the refrigerant of the refrigeration system. Thus, the amount of a particular contaminant, such as water, in the compressor drive lubricating oil is a good indication of the amount of that particular contaminant in the refrigerant of the refrigeration system.
Testing for the water concentration in the compressor drive lubricating oil is better than directly measuring water concentration in the refrigerant because it is difficult to accurately measure water concentration in the refrigerant. In a refrigeration system, refrigerant is continually changing phase and moving through the system so that the water content of the refrigerant is not constant at all locations within the system. Also, since excess water floats on top of liquid refrigerant at various places within the system, any sample of refrigerant taken from a location, other than where the excess water is present, gives an indication of water content which is lower than the actual water concentration in the refrigeration system. Thus, choosing a sampling location is of critical importance when testing directly for water in refrigerant and in practice no sampling location provides a representative sample of the refrigerant at all times. Therefore, it is particularly desirble to test the compressor drive lubricating oil for the presence of water to determine the water concentration in the refrigeration system since the oil is not subject to random water concentration variations of the type to which the refrigerant is subjected.
Tests are known for determining water concentration in compressor drive lubricating oil wherein samples of the oil are taken and chemically analyzed. However, taking samples of the oil is relatively time consuming, costly and complex. Also, a person of training is required to conduct the chemical analysis and a sample of the oil must be taken each time it is desired to make a water concentration determination. A probe for continuously and directly measuring water concentration in oil is a preferable means of making such water concentration determinations rather than conducting a chemical analysis.
Also, it should be noted that probes are known for testing a refrigerant or air to determine its water concentration. For example, a moisture sensitive indicator which changes color upon contact with water, or an electrode made of a water absorbing salt, may be used to test for water in the refrigerant of a refrigeration system. Also, ceramic sensors are known for measuring relative humidity. One such relative humidity sensor has a water sensitive probe comprising a block of porous material formed from a spinnel chrome-zinc oxide compound which is sintered with alkaline metal at high temperatures. Porous electrodes welded to lead wires are mounted on opposite sides of the porous material. Relative humidity is measured by water molecules being absorbed and released by the crystal surface of the sintered substance and by the ceramic element, whose electrical resistance varies exponentially with relative humidity. However, these probes are not suitable for determining water concentration in oil.